In many industrial and commercial fields there is a need for compact and versatile flow detectors which positively determine that a particular mass of fluid is flowing, has stopped flowing, or flows above a predetermined threshold level of flow velocity. Alternatively, such a device may be used to determine when the level of the liquid in a container has reached a predetermined height. This need is particularly strongly felt in the petroleum and commercial gas industries, which employ hundreds of thousands of miles of pipes, tubes, ducts and many other conduits to transport, and countless tanks to store enormous volumes of material in a variety of forms. These conduits and containers may be vertical, horizontal or inclined, and range in size from fractions of inches to many feet in diameter. The materials vary in composition from gases and highly reactive low-viscosity liquids to semi-solid, completely heterogeneous, corrosive mixtures of sand, mud, water and crude oil. The materials flowing in such conduits create a harsh environment.
Often the transport lines and tanks and the pumping equipment associated with them are unattended for long periods of time. Failure to note the halt in flow or the reduction below a certain predetermined flow velocity of one or more of the materials in them may be very costly and even could be catastrophic. Economic inflation and advances in technology have made, and will likely continue to make, such failures ever more costly and dangerous.
Devices have long been available for detecting and in some cases measuring, the rate of flow of fluids. The most common of these utilize the force exerted by the moving fluids against some object immersed in it to indicate or determine the rate of fluid motion. Regardless of the form chosen for the immersed object, for example, propeller, vane, piston, deflection arm, drogue or the like, all of these devices are subject to a number of serious shortcomings. Movable parts deteriorate after continued immersion for extended periods of time and become corroded or frozen in place after even brief contact with many fluids. Sealing and packing, always at least minor problems, become monumental tasks where moving parts are involved. Clogging, jamming and fouling frequently occur where the fluid contains any solids, tars or lacquers, or forms them through chemical reaction, evaporation or chemical decomposition. Mechanical deformation and fatigue induced breakdowns also plague this class of indicators. For all of the foregoing reasons, and in addition because the response rates and sensitivities in fluids of high density and viscosity are generally extremely low, particularly when these fluids are moving slowly, these devices are by and large wholly unsuitable for the detection of flow stoppage, reduction in flow velocity below a predetermined level, or changes in fluid level.
Another family of flow sensing devices operates on the Venturi principle, but these are wholly unsatisfactory for use with very dense and slow moving fluids. Furthermore, when the fluid is of high viscosity or contains solids, there is little chance of keeping the orifices, manometer tubes, bellows and other pressure sensing or conducting mechanisms free for sensing and consequently they are quickly rendered inoperative. Even when operating properly, these devices are unable to indicate positively the termination of fluid flow, or minimal changes in pressure because all of the above environmental factors influence the delicately balanced signals near and at the zero flow rate.
Examples of flow detectors employing differential temperature sensors are shown in U.S. Pat. Nos. 3,366,942 and 3,898,638. These devices have no moving parts and have proven satisfactory, at least in many circumstances where it is desirable to determine that the fluid flow has stopped. These devices, in the preferred form, generally employ a heater and two heat sensors with means for detecting differential heat responses between the sensors. The heater and the sensors are immersed in the fluid and positioned to permit the unobstructed flow of the fluid between the heater and the second sensor and are adapted so that when the fluid is flowing the heat generated by the heater and passing into the fluid is dissipated without heating either of the sensors. When the fluid is at rest the heater heats the second sensor through the fluid to a greater degree than the first sensor, thereby providing the differential temperature signal required.